In an effort to identify etiologic agent(s) in ileitis and colitis, filtrates of intestinal homogenates from patients with Crohn's disease and ulcerative colitis will be injected into footpads and peritoneal cavities of mice, and into surgically-created, accessible loops of small intestine in rabbits. The filtrates will also be cultivated in vitro in rabbit ileum and human fetal intestine tissue cultures. The animals will be followed for development of granulomas or other lesions, their immunologic status will be monitored prospectively, and the tissue cultures will be examined for virus growth. The antigenic character of cultivated virus particles will be studied by immunofluorescence and immunoelectronmicroscopy.